Nano-imprinting consists in pressing a mold into a polymer covering a substrate of silicon or another appropriate material. The mold is typically produced in silicon by standard lithography/etching techniques and is pressed into a layer of polymer heated to above its glass transition temperature so that it is deformable. After cooling and removal of the mold, the patterns of the mold are imprinted in negative form in the polymer.
To prevent potentially destructive contact between the mold and the substrate that supports the polymer, a thin residual layer of polymer is intentionally left at the bottom of the protruding patterns of the mold. The pressures applied to the mold are such that, if the mold and the substrate were to come into direct contact, the two wafers would be weakened and could break. To guarantee the presence of this residual layer, the initial polymer thickness is chosen so that, at the end of pressing, the polymer fills the recesses of the patterns of the mold.
The residual polymer thickness is then eliminated by an oxygen plasma, which locally exposes the substrate. The patterns of the polymer layer are then reproduced in the substrate (transferred thereto) by plasma etching (typically by reactive ion etching (RIE)), as in the usual lithography/etching situation.
A technique of the above kind is described in the document “Imprint of sub-25 nm vias and trenches in polymers” by S. Y. CHOU, P. R. KRAUSS, and P. J. RENSTROM, published in Appl. Phys. Lett. 67 (21) 20 Nov. 1995, pp. 3114-3116.
The main difficulty of the above technique is obtaining a uniform residual thickness regardless of the size and the density of the pressed patterns. If the residual thickness at the bottom of the pattern is not homogeneous, the oxygen plasma that is intended to eliminate it will induce a local modification of the size of the patterns where the layer is thinnest; because the patterns are unknown a priori, this reduction in size cannot be taken into account in determining the dimensions of the patterns of the mold. This leads to inaccurate control of the dimensions of the patterns, which is incompatible with industrial use of this technique.
This erratic modification of the size of the patterns of the polymer layer may be explained as follows.
Firstly, the local differences in the residual thickness result from the fact that, the closer together the protrusions and the recesses of the pattern of the mold, the more the penetration of the mold into the polymer layer implies the displacement of a significant quantity of material, and the more difficult it is for the mold to “enter” the polymer layer.
During the step of oxygen plasma etching of this residual layer at the bottom of the pattern, the polymer material is eliminated everywhere in the direction of the substrate. However, if the underlying surface of the substrate has been exposed in a location where the residual layer is originally very thin, continued application of the plasma (which is necessary to eliminate the residual layer where it is thickest) results in now lateral attacking of the polymer, reflected in a localized enlarging of the recesses of the pattern. As a consequence of this, the most isolated patterns (where the residual layer was thinnest) are reduced in size relative to the protrusions of the molds, whereas the most dense patterns (where the residual layer was thickest) remain strictly identical to the protrusions of the mold.
This thickness disparity is very difficult to avoid. The main pressing parameters are the pressing pressure, temperature and time. Tests have shown that, to etch lines 500 nm wide with a space between the lines that varies from 650 nm to 10 000 nm at a pressure of 50 bar at a temperature of 120° C., the residual thickness varies from 55 nm to 120 nm for a pressing time of 5 minutes, from 40 nm to 75 nm for a pressing time of 30 minutes and from 65 nm to 75 nm for a pressing time of 60 minutes. This demonstrates that the thickness disparity (the maximum thickness is generally observed for a distance between the lines of the order of 1000 nm) and that this disparity is reduced if the pressing time is increased.
It is therefore apparent that it is possible to obtain substantially uniform pressing in an array but that this implies temperatures and times that may appear too high and too long and therefore too costly (the higher the temperature, the shorter the pressing time needs to be).
This makes the above technique slower and therefore less advantageous than the standard methods.
However, the foregoing description relates to an array of given size, with particular patterns (made up only of lines, of the same width) and it may be concluded that although it is possible with well-adapted conditions to obtain an array of lines uniformly pressed in given sizes, it is virtually impossible under the same conditions to obtain a residual thickness of the same value in arrays with different pattern sizes and densities, and a fortiori with patterns of diverse shapes. Consequently, the complete investigation of homogenization of pressing has to be repeated as a function of the three parameters cited above as soon as the size of the array or the pressing patterns is changed.
To obtain imprints of good quality, it has been proposed, in particular in the document “Tri-layer systems for nano-imprint lithography with an improved process latitude” by A. LEBIB, Y. CHEN, F. CARCENAC, E. CAMBRIL, L. MANIN, L. COURAUD and H. LAUNOIS, published in Microelectronic Engineering 53 (2000) 175-178, to employ a technique using three layers on the substrate to be etched: this substrate is covered with a lower layer of PMGI resin cured at 270° C., in turn covered with a thin layer of germanium, in turn covered with an upper layer of PMMA or S1805 resin. The method comprises multiple steps: pressing the mold into only the upper layer, eliminating the residue of the imprinted portion of the upper layer, and transferring the pattern into the germanium layer by attacking this intermediate layer using the upper layer as a mask, this germanium layer thereafter serving as a mask for attacking the lower layer. This is followed by the deposition of a metal layer on the lower layer followed by elimination of the lower layer: the portions of the metal layer that were at the bottom of the recesses of this layer, on the surface of the substrate, are the only ones to remain and finally serve as a mask for attacking the substrate. Note that this method is of a different kind from that described above, since it is not the lower layer whose protruding portions serve as a mask for attacking the substrate, but rather the metal portions deposited directly on the substrate, corresponding to the recesses of the lower layer. This technique, involving deposition at the bottom of the recesses of the lower layer, is often called the “lift-off” technique.